Antibodies as therapeutics is one of the most rapidly growing segments of biopharmaceuticals in which proteins are used as active pharmaceutical ingredients. The antibody field of research and development is broad and includes the use of full length antibodies (Ab), single chain variable region fragments (scFv), single domain antibodies (dAb) and other multi-functional constructions (eg diabodies etc) (Presta, L G. Curr. Opin. Immunol. 2008, 20:460-470). The main disease targets for antibody therapies are cancer, autoimmune diseases, cardiac diseases and infectious diseases.
One of the major challenges of antibody production is faced during work-up after fermentation which may use eukaryotic (yeasts, plants or mammalian) or prokaryotic (bacteria) expression vehicles depending on the type of antibody construct under study. Numerous costly purification steps have to be employed in order to reach the required purity levels. These often consist of combinations of two or more different separation techniques (Satinder Ahuja (Ed.), Handbook of Bioseparations in Separation Science and Technology (Vol. 2). Academic Press, San Diego Calif., 2000) aiming at removing impurities to reach an acceptable purity level. A typical process would involve:

So far no single separation technique capable of taking crude antibody through to late stage filtration exists. As shown above, the downstream process is commonly divided into a capturing and separation step and a various polishing steps, each with their different requirements on the respective separation materials. Conventionally the choice of separation technique used depends on cost, binding capacity and reusability.
The capturing and initial separation step is typically based on affinity materials such as protein A, small ligands, aptamers etc. The most commonly used affinity material is protein A attached to a solid support such as agarose, silica or dextran. Protein A is a relatively robust 42 kD protein exhibiting affinity for the Fc region (heavy chain) of IgG (Immunoglobulin G)—see FIG. 1. While commonly used, Protein A materials are expensive, may result in column bleeding and consequential toxicity problems if not removed and may undergo denaturation during regeneration which typically involves high pH cycling. They are also susceptible to protease digestion and concomitant loss of function.
Alternative, less selective chemical materials can be used such as those based on thiophilic, boronate, immobilized metal ion chromatography (IMAC) or hydrophobic interaction chromatography (HIC) principles. All these methods suffer from different drawbacks (requirement of additional, non-native amino acids to be present at the C-terminus of the antibody chain (eg oligo-histidine for IMAC), leakage, need for high salt, low selectivity etc) which have precluded their widespread use at the process scale level. Recent improvements however, have been achieved by combining two separation principles in a single ligand incorporating ion-exchange and thiophilic separation principles.
For the intermediate/polishing purification steps either multi-modal anion exchange resins with or without further anion or cation exchanger steps may be employed (eg Capto Adhere, CaptoQ or CaptoS from GE Healthcare). While such steps are currently necessary the sum total adds up to 3 to 4 different stages of purification each of which will contribute to antibody losses and each of which needs to fulfill the regulatory (eg FDA) requirements of being stable to alkaline regeneration without leakage of components into the antibody preparation.
Product Polishing describes the final processing steps which end with packaging of the product in a form that is stable, easily transportable and convenient. Crystallization, desiccation, lyophilization and spray drying are typical unit operations. Depending on the product and its intended use, polishing may also include operations to sterilize the product and remove or deactivate trace contaminants which might compromise product safety. Such operations might include the removal of viruses or depyrogenation.
US2003049870 mentions that molecularly imprinted polymers may be prepared using entire antibodies as templates. The potential advantages of MIPs used in this way are however severely compromised where the template (an antibody) is the same molecule as the therapeutic product (viz the antibody). Further, an intact antibody as a template is expensive to produce and adds significant cost to the MIP thereby making it less competitive than existing methods (eg Protein A). It is also liable to retention in the polymer and hence leakage during use.